Electronic devices often contain displays. A display may have an active area with pixels that display images for a user and an inactive area running along the edge of the active area. A layer of glass may serve as a protective display cover layer. The layer of glass may overlap the active area and the inactive area. To hide internal components from view, the inner surface of the inactive area may be covered with an opaque masking layer such as a layer of black or white ink. Windows in the display cover layer may be formed from openings in the opaque masking layer. Light-sensitive components may be aligned with the windows. For example, an infrared proximity sensor may be aligned with a window in a display.
Infrared proximity detectors may contain light emitters such as light-emitting diodes that emit infrared light. Some of the emitted infrared light may be reflected off of external objects in the vicinity of an electronic device. A light detector in an infrared proximity detector may measure the reflected infrared light to produce a proximity sensor output signal. The magnitude of the proximity sensor output signal is indicative of whether or not an external object is located in close proximity to the electronic device.
To hide an infrared proximity sensor from view, a coating layer may be used to coat a window aligned with the proximity sensor. The coating layer may be formed from a printed ink layer that is transparent at infrared wavelengths. The printed ink, which may sometimes be referred to as infrared ink, may be opaque at visible wavelengths and may have a black appearance that approximates the appearance of the opaque masking layer in the inactive area or other suitable appearance.
Although use of a printed black infrared ink to cover a proximity sensor window may help improve the outward appearance of the electronic device, printed ink coatings may have rough surfaces and other light-scattering features. As a result, printed ink coatings may scatter light emitted by infrared light-emitting diodes in proximity sensors. The scattered light can lead to crosstalk from undesired back reflections from the display cover layer. The outwards appearance of the infrared ink may also not be well matched to the surrounding black ink in the inactive area of the display.